Stainless steel is a suitable material for sewing needles to be used in surgical operations. In the case of martensitic stainless steel, precipitation hardening stainless steel, or the like, wire rods each 6 to 10 mm in diameter are used as the material. In the case of carbon steel or martensitic stainless steel, wire drawing is carried out repeatedly thereto, providing wires each having the diameter of a sewing needle being fabricated. In the case of stainless steel, a solution heat treatment is carried out between each wire drawing.
These wires are cut at an appropriate length, and tips thereof are then sharpened by grinding using a grindstone into a cone or pyramid shape, which is then incurvated through machining and a hole passing through from a base end towards the tip side is made using a drill or a laser. Afterwards, the tip is mainly quenched, and a suture thread is inserted in the above-mentioned hole, and crimped and fixed. In the case of precipitation hardening stainless steel, once wire drawing and the solution heat treatment are carried out repeatedly until the thickness of the sewing needle is reached, it is cut at a predetermined length, the tip is sharpened as mentioned above, and then a hole is made in the base end. Afterwards, precipitation hardening other than quenching is carried out.
Since the above materials are soft, processing is easy, but there is a problem with quenching and precipitation hardening in that cracking, breaking, chipping, or the like easily occurs due to lack of toughness. Furthermore, there is a problem with corrosion resistance since rust easily develops due to characteristics of the materials.
With regard to these problems, Japanese Patent Publication after examination (Kokoku) No. Hei 1-11084 proposes a fabrication method for completing a product by using austenitic stainless steel wires extended at an 80% or greater cross-sectional area reduction rate, and in subsequent processing, carrying out predetermined processing on these steel wires while always maintaining at a temperature below approximately 500° C. Since hardness increased through work hardening, or strain hardening decreases when exceeding 500° C., the temperature is set to no greater than 500° C. While austenitic stainless steel cannot be quenched, work hardening at the time of wire drawing is utilized instead. Utilization of austenitic stainless steel allows improvement in corrosion resistance. Furthermore, since quenching is impossible, the problem of cracking or chipping does not occur by contrast.
The austenitic stainless steel extended to a predetermined diameter in this manner becomes a thin fibrous structure with crystal grains extending along the line length, providing the necessary hardness for a sewing needle. In this case, these stainless steel wires are cut to a predetermined length, tips thereof are sharpened into a conical or pyramid shape, and then a hole is made in the base end side using a drill or laser.
However, since the entirety from the tip to the base end is a fibrous structure, it is hard, and processing such as crimping to attach a suture thread to the sewing needle is difficult. Furthermore, the hole cracks even due to crimping, causing damage to the anatomy, which thereby reduces tension of the suture thread, making it easier to pull out. Accordingly, Japanese Patent Publication after examination (Kokoku) No. Hei 4-67978 proposes to heat the formed hole area using a burner flame, electrical resistance, or high-frequency induction to make it a structure without long crystal grains, soften it, and then carry out a crimping operation.
However, while the sewing needle described in Japanese Patent Publication after examination (Kokoku) No. Hei 4-67978 is formed with the hole included needle base having a granular crystalline structure without any directionality, the granular structure extends much further beyond the hole towards the needlepoint. Therefore, if a surgeon grips near the hole of the sewing needle when suturing, there is a problem that he/she grasps a flexible granular structure, resulting in a bent sewing needle. In this case, the needle should be gripped on the tip side, for example, a position approximately a third of the needle length from the needle base; however, depending upon suturing portions, there is a case where it is easier to suture by gripping closer to the hole, for example, a position approximately a quarter to a fifth of the needle length from the needle base. Needing to always grip on the tip side of the needle is stressful for the doctor.
Note that a tendency to grip the tip side of the needle at a position approximately three times the diameter of the sewing needle from the bottom of the hole is understood.